1. Field of the Invention
The present invention relates to a magneto-optical recording medium for recording or erasing information by utilizing an increase in temperature caused by irradiation with laser light and for reading a recorded signal by utilizing a magneto-optical effect; a method of manufacturing the magneto-optical recording medium; a method of recording on the magneto-optical recording medium; and a method of reproduction from the magneto-optical recording medium.
2. Related Art of the Invention
Optical memories capable of reproduction of information by detecting reflected light from a irradiated light beam. Types of optical memories include a read-only memory (ROM) in which information is recorded as phase pits, and a write once type of optical memory in which holes are formed in a recording film by irradiation with a light beam to record information. Other types include a phase change type of optical memory in which a crystalline phase in a recording film is changed by irradiation with a light beams to record information, and a magneto-optical memory in which the direction of magnetization of a recording layer is changed by irradiation with a light beam and application of a magnetic field to record information.
In these optical memories, the resolution at which a signal is reproduced is virtually determined by the wavelength λ of reproduction light and the numerical aperture (N. A.) of the objective lens, and a detection limit pit cycle is approximately λ/[2·(N. A.)].
However, it is difficult to reduce the wavelength of reproduction light or increase the numerical aperture of the objective lens. Attempts have therefore been made to increase the information recording density by devising a recording medium or a reproduction method.
In particular, various means or methods for increasing the information recording density in magneto-optical recording mediums on a trial basis have been proposed.
For example, a technique is known which improves the reproduction resolution exceeding the above-described detection limit determined by the wavelength and the numerical aperture of the objective lens in such a manner magnetic domain walls coming near to a reproducing light beam are successively displaced and the displacement of each domain wall is detected (see Japanese Patent Laid-Open No. 6-290496).
The disclosure in Japanese Patent Laid-Open No. 6-290496 is incorporated herein by reference in its entirety.
In this technique, a particularly improved reproduction signal can be obtained if a reproduction layer which is a first magnetic layer in which each of magnetic domain walls is displaced when it comes near to a reproducing light beam is magnetically separated between each adjacent pair of information tracks.
Needless to say, in a case where microscopic recording magnetic domains in a recording layer in which information is recorded at a high density are transferred into a reproduction layer, and where the recorded information is reproduced by displacing domain walls in the reproduction layer or by a like method, it is necessary that the microscopic magnetic domains in the recording layer be maintained with stability and strongly and stably transferred into the reproduction layer by magnetic coupling.
However, the vertical magnetic anisotropy of the recording layer can be reduced, depending on the composition of the recording film and the film forming method. In some cases, therefore, it is difficult to form microscopic recording magnetic domains in the recording layer with stability.
Also, stable magnetic coupling using the vertical anisotropy of the recording layer is necessary for transfer of recording magnetic domains from the recording layer into the reproduction layer. In a case where the transferability changes depending on magnetic characteristics of the recording layer so that the stability of transfer is low, transfer noise and noise accompanying a domain wall displacement may be increased to such an extent that the reproduction signal quality is considerably reduced.
To effect a domain wall displacement with stability, a method of effecting magnetic separation between information tracks by laser annealing, a method of effecting shutoff between information tracks by using an optical disk substrate having a land/groove configuration, or the like is used. However, the characteristics of transfer from the recording layer into the reproduction layer may change, depending on laser annealing conditions or the land/groove configuration of the optical disk substrate, to such an extent that the influence of groove noise from the optical disk substrate is considerably large.
In particular, if the groove depth is large or the groove width is small in the case of groove recording, the shape of recording magnetic domains may change, depending on the microscopic structure of recording layer, to such an extent that the amount of signal in reproduction by transfer into the reproduction layer is reduced considerably.
As described above, there is a possibility of failure to obtain a sufficiently high recording density or transfer rate in magneto-optical recording.
In view of the above-described problems of the conventional art, an object of the present invention is to provide a magneto-optical recording medium capable of improving the recording density and the transfer rate in magneto-optical recording, a method of manufacturing the magneto-optical recording medium, a method of recording on the magneto-optical recording medium, and a method of reproduction from the magneto-optical recording medium.
The present invention can provide a magneto-optical recording medium capable of improving the recording density and the transfer rate in magneto-optical recording, a method of manufacturing the magneto-optical recording medium, a method of recording on the magneto-optical recording medium, and a method of reproduction from the magneto-optical recording medium.